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Usually variables are graphically set trough through the preferences window, but some unusual variables must be set from the lower command line, with this command:
Mescape Utilities Variables <varname> <varvalue>
if <varname> is not specified then the mouse contextual menu (see Mouse operations) will show the list of available variables.
or with the Tcl GiD_Set scripting command
to ask the value
GiD_Set <varname>
to set the value
GiD_Set <varname> <varvalue>